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I own a 24x60' uninsulated building that's about 100 years old. The roof is matched 2x4 rafters (no ridge board) with 2x4 ceiling joist (technically rafter ties I guess). As you can imagine, the joists are sagging quite substantially. Previous owners have tried to fix it with a mix of various methods. I want it fixed one and for all because I'm planning on drywalling and insulating the building.

I don’t think I can put in bigger joist because there would be no room to fit them at the top plate. Definitely not the 2x12's that would be required. I'd really rather not put in posts and beam because the ceiling is only 8' high.

My thought is to jack a joist straight, sister a 4’ 2x4 to it in the middle, tie the now straight joist to a strongback. Would this provide enough support to hold the sag out and support drywall and insulation?

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    A couple of photos would really help people visualise the situation and problem.
    – handyman
    Commented Dec 30, 2020 at 23:04
  • you said "jack the joist straight" . did you mean jack the rafter straight? also how long are your rafters and spacing?
    – ojait
    Commented Dec 31, 2020 at 1:25
  • The post said that the ceiling joists (rafter ties) are sagging, not the rafters. They'd need to be lifted. We can assume that the shed has about a 4:12 pitch, so rafter length is roughly 13'.
    – isherwood
    Commented Dec 31, 2020 at 1:27

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You have several challenges when adding many thousands of lbs. of weight to a roof system like that (which almost certainly wasn't designed with that use in mind):

  • The ceiling joists aren't up to the task, as you know. The only real fix for this over a 24' span is to tie them to the rafters, which leads us to the next problem...
  • The rafters are barely adequate as they are. If you now hang the weight of a gypsum ceiling on them they'll soon be looking like your ceiling joists do, or they'll just give up the ghost. Unless...
  • You replicate a modern engineered truss with V-webbing and adequate gussets at all connections. They'd look about like this when done:

enter image description here

source

Note that gussets must be fastened properly--those spiky steel plates cannot be simply pounded into place. They're not designed for that. If you were to screw or nail plywood gussets on, they'd need to be robust enough. Wood isn't as strong as steel, so you need a larger panel of suitable thickness.

As to your strongback proposal... a strongback stiffens things by sharing load across several attached members and keeps them on plane. It does not hold them up. You'd have to install a beam that spans the 60' down the center of your building while carrying nearly all the weight of your insulation and drywall.

I strongly urge you to get a local expert on scene. This is a project that could get dangerous in a hurry if not done well. Good luck.

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  • I actually considered doing exactly this. The only problem is the joists are out of plane with the rafters. The rafters sit on the top plate The joists sit on the top plate next to them and are nailed into the side of the rafters. If I made trusses the webbing would have to be tilted which I assumed would keep it from working.
    – Jack Baker
    Commented Dec 30, 2020 at 22:33
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    There are truss designs with a doubled bottom chord - you'd add an additional chord to the opposite side of each rafter pair, along with the webs and appropriate connectors. You should get engineering help to actually design a system (which you can build to their design) that would work "for sure" rather than cooking one up yourself, since you want it "fixed once and for all."
    – Ecnerwal
    Commented Dec 30, 2020 at 23:05
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    I didn't know that but it seems like exactly what I need. I'll call a truss engineer tomorrow and see if they're willing to check out what I'm dealing with and design something with a double bottom coard. Thanks for y'all's help.
    – Jack Baker
    Commented Dec 31, 2020 at 1:23
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Consulting an engineer would be your best option as stated in another post. In addition to that, have you considered using a different interior finishing material than drywall? The same steel for siding pole barns can end up being both a quicker, neater, and in many cases a cheaper option for interior finishing. It is vaporproof and by using sealant on two sides can negate the need to install a vapor barrier or have to use special insulation like kraft-faced rolls. It doesn't require as much support so less materials are necessary. It comes prefinished and in longer lengths (16, 20, 24, 30') meaning less time installing. A 3 man team using two sheetrock jacks could do the ceiling then walls in under a day after framing is complete.It will not collect odors and is impervious to humidity.

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